German patent application DE 198 44 843 A1 discloses a special planetary gear train with so-called “stepped planets”. Here, planet gears of different sizes are arranged in pairs in two radial planes that are at an axial distance from each other, whereby the larger planet gears mesh only with the sun gear while the small planet gears mesh only with the annulus. The two planet gears of each stepped planet are coupled via a shaft so as to be non-torsional. Each planet gear is mounted axially at the front and back of its gear body in a roller bearing, especially in a cylindrical roller bearing. This translates into a highly stressable, speed-resistant, low-friction and stiff bearing. Of course, this presupposes a correspondingly precise and sturdy planet gear carrier so that the load-bearing capacity of the bearings and toothing can actually be utilized.
The fact that each planet gear is mounted on both sides in a roller bearing is also advantageous for “simple” planetary gear trains of the above-mentioned type if large amounts of power are to be transmitted with low friction.
Even though roller bearings are bigger and more voluminous than slide bearings of a comparable load-bearing capacity, their advantages are that they entail little friction, even at low speeds, that they have good anti-seizing properties in case of insufficient lubrication and that they can tolerate dirt better.
In order to mount the planet gears on bearings on both sides, it is common practice to configure the planet gear carrier with two axially spaced bearing washers as well as with webs that connect the bearing washers. The webs are positioned in the gaps between the planet gears so that the number of webs usually corresponds to the number of planet gears. Here, the objective is to always connect the webs to the bearing washers so as to create the most dimensionally stable possible “cage”. It is also a known procedure to apply and withdraw the force and torque on the planet gear carrier in its axial center via the webs in order to prevent unwanted deformations or asymmetries.
In spite of all of these measures, manufacturing tolerances, especially in the area of the planet gears and of the planet gear carrier, can give rise to local load peaks on the toothing and bearings, thus reducing the transmittable power and the service life of the gear train.